Fantastic video explaining Diffie-Hallman key exchange alogrithm used in asymetric gyptography in a simple way.
https://www.youtube.com/watch?v=YEBfamv-_do
Thanks to Khan academy for yet another lessions.
This video explain the RSA algorithme https://www.youtube.com/watch?v=Z8M2BTscoD4
And her you find an example where the Euler Totient function is calculated in an other way https://www.youtube.com/watch?v=kYasb426Yjk
For sysetric encryption I find this video wery good https://www.youtube.com/watch?v=jKkZ9-Ij6Hs